When milk is churned,cream gets separated due to

$Assertion$: Use of ball bearings between two moving parts of a machine is a common practice.
$Reason$: Ball bearings reduce vibrations and provide good stability.

$A$ $20 \text{ ton}$ truck is travelling along a curved path of radius $240 \text{ m}$. If the center of gravity of the truck above the ground is $2 \text{ m}$ and the distance between its wheels is $1.5 \text{ m}$, the maximum speed of the truck with which it can travel without toppling over is (Acceleration due to gravity $= 10 \text{ ms}^{-2}$) (in $\text{ ms}^{-1}$)

The upper half of an inclined plane of inclination $\theta$ is perfectly smooth,while the lower half is rough. $A$ block starting from rest at the top of the plane will again come to rest at the bottom if the coefficient of friction between the block and the lower half of the plane is given by:

$A$ helicopter of mass $1000 \;kg$ rises with a vertical acceleration of $15\; m s^{-2}$. The crew and the passengers weigh $300\; kg$. Give the magnitude and direction of the
$(a)$ force on the floor by the crew and passengers,
$(b)$ action of the rotor of the helicopter on the surrounding air,
$(c)$ force on the helicopter due to the surrounding air.

On which factors does the stopping distance of a vehicle depend?